Use of sulfamic acid in chlorination step of multistage bleaching process



FIPESOE United States Patent This invention relates to the bleaching ofpaper pulp. More particularly, this invention refers to an improvementin the use of sulfamic acid in paper pulp bleaching operations carriedout with chlorine, hypochlorite or hypochlorous acid.

The main objective in bleaching wood pulp is to obtain a white pulp withstable color and with a minimum deleterious effect on physical andchemical properties of the pulp.

Wood pulp such as kraft, sulfite, etc., for use in making paper iscustomarily bleached in a series of steps, generally referred to as theMultistage Bleaching process. This process involves pulp bleaching andpurification wherein the various impurities in the fiber are removed ina gradual and orderly way in a series of stages without seriousdegradation of the fiber in any one stage.

The first stage is a chlorination of the pulp at very low pH, such as 1to 2. In this first stage most of the residual lignins are chlorinated,ordinarily using from 5 to 7% chlorine concentration, expressed asavailable chlorine based on the weight of the pulp. The second stageinvolves a caustic extraction wherein the substitution products formedin the first stage are solubilized and washed out. The third stage orhypochlorite stage is a true bleaching stage wherein hypochloritesolutions at pH between 8 and 11 are employed. Ordinarily, hypochloriteconcentration from 0.5 to 2%, expressed as available chlorine based onthe weight of the pulp, and temperatures of 30 to 40 C. for a period of1 to 5 hours will be used in the third stage.

According to the present invention, I have found an improvement in pulpbleaching operations whereby one can obtain outstanding pulp brightnesswith minimum degradation or loss of strength of the pulp.

In the present invention, a critical amount of sulfamic acid is admixedwith the pulp being treated in the first stage of the pulp bleachingoperation. The amount will be from 5 to 20% by weight, based on theweight of the available chlorine in the first stage operation.

For some reason, the use of this small but important amount of sulfamicacid in the first bleaching stage helps to produce pulp of excellentbrightness and improved tear, bursting and tensile strength.

In a preferred aspect of the present invention, sulfamic acid in acritical amount is also used in the third stage of the pulp bleachingoperation. The amount will be from 1 to 30% and preferably 5 to 20% byweight, based on the weight of the available chlorine in the thirdstage.

In this third stage, temperatures of 30 to 80 C. are satisfactory but Iprefer a temperature in the range from 55 to 75 C. A pH of 6 to 11 canbe used. Bleaching times of 1 to 3 hours will normally be employed.

Instead of sulfamic acid itself, an alkali or alkaline earth or ammoniumsalt can be used. Sodium, potassium, calcium, lithium and ammonium saltsare particularly useful. Amounts should be calculated on an acidequivalent basis.

It is surprising that, using the small but critical, amount of sulfamicaccording to this invention, one can obtain high brightness pulps havingan improved viscosity of 200% or higher compared with pulp treatedidentically except without the use of the specified amount of sulfamic.It is generally recognized that there is a direct correla- "ice tionbetween viscosity and tear, bursting and tensile strength. A standardstrength test involves dissolving the pulp in cupriethylene diamine andmeasuring the viscosity of the resulting solution (TAPPI Standard T230Sm-SO). Brightness can be measured according to standard tests (TAPPIStandard T-217 657 m-48).

If desired, the pulp can be additionally treated with a secondhypochlorite stage and/or with other oxidizing agents such as hydrogenand/ or sodium peroxide, chlorine dioxide, and the like, as will bereadily understood.

This invention will be more fully understood by reference to thefollowing illustrative examples:

Example 1 Thirty grams of unbleached oven-dried paper pulp are mixedwith 2,000 milliliters of water and disintegrated in a Herrmanndisintegrator for 30 minutes. The pulp is then filtered through aBuchner funnel and placed into a polyethylene bag. Separately, 40milliliters of aqueous chlorine solution (2 grams of available chlorine)are dissolved in 760 milliliters of water, mixed with 0.2 gram ofsulfamic acid, and added to the pulp. The bag is closed with a rubberband and kneaded by hand for several minutes to insure uniformdistribution of the contents. A consistency of about 4% and a pH ofabout 2 is obtained. The bag is kept in a constant temperature waterbath at 22 C. for 60 minutes, repeating the kneading every 10 minutes.

The pulp is removed from the bag, filtered in a Buchner funnel, andplaced in an empty polyethylene bag where it is mixed with a solution of2 grams of sodium hydroxide dissolved in 250 milliliters of water(consistency of about 12%). The bag is then closed as before, kneadedfor 2 minutes and placed in a water bath at 6080 C. for 1 hour,repeating the kneading every 10 minutes.

The pulp is removed from the bag, rinsed with distilled water, filteredin a Buchner filter and placed in an empty polyethylene bag. Separately,l5 milliliters of sodium hypochlorite solution (0.75 gram of availablechlorine which is 2.5% of available chlorine based on the weight of thepulp) are dissolved in 200 milliliters of distilled water and added tothe pulp. Drops of sodium hydroxide solution are added to obtain a pHbetween 10 and 11. The bag is closed with a rubber band, kneaded for 2minutes and place-d in a water bath at 60 C. for 90 minutes, repeatingthe kneading every 10 minutes.

The pulp is removed from the bag, filtered in a Buchner funnel, andwashed with distilled water until a neutral pH is obtained in thefiltrate. Pulp brightness and strength are excellent, as determined bystandard tests.

Example 2 The preceding example is repeated except that 0.15 gram ofsulfamic acid is admixed with the hypochlorite solution in the thirdstage of the operation. By these procedures, a bleached paper pulp hasbeen readily obtained having a viscosity of 18.9 centipoises (0.5% Cu Edviscosity brightness of 77.86 GB (TAPPI Standard T-217 m-48).

The invention claimed is:

1. In the multiple stage chlorination bleaching of wood pulp whichinvolves a first stage chlorination bleach of the pulp at pH 1-2, asecond stage extraction with caustic, and a third stage hypochloritebleach at pH 611 and a temperature of 30 to C. for 1 to 3 hours, theprocess improvement of admixing with the pulp in said first stage bleachfrom 5 to 20% by weight of a compound selected from the group consistingof sulfamic acid and its alkali, alkaline earth and ammonium saltscalculated on an acid equivalent basis and based on the weight of theavailable chlorine in said first stage bleach.

2. The process as set forth in claim 1 wherein there is admixed with thepulp in said third stage bleach from 1 to 30% by weight of a compoundselected from the group consisting of sulfamic acid and its alkali,alkaline earth and ammonium salts, calculated on an acid equivalentbasis and based on the weight of the available chlorine in said thirdstage bleach.

3. The process as set forth in claim 2 wherein said compound is sulfamicacid.

4. The process as set forth in claim 2 wherein said 10 compound issodium sulfamate.

5. The process as set forth in claim 2 wherein said compound is ammoniumsulfamate.

5 5 to 20% by weight, calculated as specified.

References Cited by the Examiner UNITED STATES PATENTS 3,177,111 4/1965Larsen 16273 DONALL H. SYLVESTER, Primary Examiner.

HOWARD R. CAINE, Examiner.

1. IN THE MULTIPLE STAGE CHLORINATION BLEACHING OF WOOD PULP WHICHINVOLVES A FIRST STAGE CHLORINATION BLEACH OF THE PULP AT PH 1-2, ASECOND STAGE EXTRACTION WITH CAUSTIC, AND A THIRD STAGE HYPOCHLORITEBLEACH AT PH 6-11 AND A TEMPERATURE OF 30* TO 80* C. FOR 1 TO 3 HOURS,THE PROCESS IMPORVEMENT OF ADMIXING WITH THE PULP IN SAID FIRST STAGEBLEACH FROM 5 TO 20% BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUPCONSISTING OF SULFAMIC ACID AND ITS ALKALI ALKALINE EARTH AND AMMONIUMSALTS CALCULATED ON AN ACID EQUIVALENT BASIS AND BASED ON THE WEIGHT OFTHE AVAILABLE CHLORINE IN SAID FIRST STAGE BLEACH.